Process for converting hydrochloric acid into chlorine



Oct. 7, 1958 H. J. WALTER PROCESS FOR CONVERTING HYDROCHLORIC ACID INTOCHLORINE Filed March 29, 1956 WW. INVENTOR.

nited States Patent ce PROCESS FOR CONVERTING HYDROCHLORI ACID INTOCHLORINE This application is a continuation-in-part of my copendingapplication Serial No. 389,894, filed November 2, 1953 (now abandoned)and relates to a process for converting hydrochloric acid into elementalchlorine by oxidizing said hydrochloric acid in aqueous solution withexcessive quantities of oxygen or air in the presence of nitrogendioxide or nitric acid, whereby the temperature of the reaction batch iskept between 50 C. and 100 C. or higher, however such, that the reactionmixture is substantially maintained in the liquid state. The gaseousmixture of chlorine together with oxides of nitrogen and inert gases aresubsequently scrubbed with sulfuric acid of concentrations between 60%and 98%, whereupon pure chlorine is recovered by extraction with carbontetra chloride under pressure. Extremely high yields and throughput maybe obtained by decreasing the stay time inside the reactor to such anextent as to obtain an exhausted hydrochloric acid of about 3% to andsubsequently passing this diluted hydrochloric acid to any point forre-loading with hydrogen chloride, as occuring in chlorination, wherebythe enriched aqueous hydrochloric acid is recycled to the converter.

The recovery of chlorine from hydrochloric acid' is of specialimportance for processing hydrogen chloride, occuring in largequantities in all chlorination processes. Most suggestions for recoveryof chlorine from hydrochloric acid are based on the Deacon principle. Itwas likewise suggested to perform said recovery by oxidizing hydrogenchloride by means of nitric acid. All these processes have common thatdehydrated gaseous hydrogen chloride can be processed exclusively. Butsince hydrogen chloride can practically be stored, shipped or otherwisetransported in aqueous solutions only, all hitherto known processes hadto provide previous re-dehydration of hydrochloric acid, previous tooxidation.

My present invention avoids said disadvantages, since it is based on animmediate processing of aqueous hydrochloric acid of any concentration.My new process consists essentially in oxidizing said liquid aqueoussolution of hydrochloric acid with oxygen orair in the presence ofnitrogen dioxide, nitric acid, nitrosyl chloride or nitric oxide attemperatures of at least 50 C. Nitrogen dioxide will in generalconstitute the main oxidation catalyst, since nitric acid reacts largelywith hydrochloric acid according to the aqua regis-reaction:

2HNO +6HCl=4H O+2Cl +2NOCl However nitrosyl chloride, the decompositionof which was a serious problem in known processes, is readily de- Nitricoxide is immediately oxidized by present air or oxygen to nitrogendioxide according to:

i atented Oct. 7, 1958 The latter oxidizes hydrochloric acid accordingto the following equation, which is prevailing in my process:

Nitric oxide however is immediately oxidized to nitrogen dioxide, asshown previously. An important feature of the new process is to provide,that the reaction is run at elevated temperatures. In generaltemperatures between 50 C. and 110 C. are appropriate. Experimentsshowed that the reaction rate and throughput are greatly increased byincreased temperatures. Preferred temperatures for the process arebetween C. and C. On the other hand it is not advisable to run thereaction too close to the boiling point on reason of preventing undueevaporation and heat losses, that can no longer be compensated by heatexchangers. Of course it is possible to provide temperatures higher thanC., if the reaction is performed under increased pressure.

It is noteworthy to state that the above described reactions could notbe foreseen. Thermodynamical calculations showed that the aboveequations run endothermic, on reason of the caloric data given inliterature. This however holds for low temperatures only. At elevatedtemperatures it was surprisingly found, that the above simultaneousreactions are running exothermic. This is presumedly due to the fact,that the fugacity of hydrogen chloride is greatly increased withincreasing temperatures.

Comparative tests showed furthermore, that reaction rate and throughputcan be considerably improved by providing reactors with packingmaterial. Any material, sufiiciently resistant against the attack of hotmuriatic acid, may be used. However I give the preference to coke,graphite, both in lumps or any other form like Raschig rings, pebbles orBerl saddles. Phenoplasts, chlorinated rubber, hardened rubber, polyvinyl chloride or Teflon are likewise useful. Same material provedsuccessful for lining the reactor walls. Said packing material does notonly provide increased surface but likewise stimulates the equilibriumbetween the liquid and the gaseous constituents inside the reactor, in amanner specific for the new process.

The gases issuing from the reactor consist of chlorine, nitrogendioxide, excessive oxygen and possibly atmospheric nitrogen, carryingalong some moisture. These gases are then scrubbed with sulfuric acid,containing 70% to 98% of H 80 By this way nitrogen dioxide as well asmoisture are absorbed. It is known that concentrated sulfuric acidabsorbs nitrogen dioxide, and it is furthermore well known thatconcentrated sulfuric acid also absorbs water vapor. However it islikewise known, that nitrogen dioxide, absorbed in concentrated sulfuricacid with the formation of nitrosyl sulfuric acid, is liberated again bythe addition of water. It was therefore surprising, that sulfuric acidaccording to the present invention, is capable to absorb simultaneouslyboth, water and nitrogen dioxide, as occuring in the the new process.Consequently the sulfuric acid scrubber can be operated with sulfuricacid of e. g. 80% to 70% and even as low as 60%.

The sulfuric acid loaded with nitrogen dioxide and water is subsequentlyheated up in order to expel nitrogen dioxide and water, whereby thesulfuric acid is re-concentrated up to some concentration between said60% and 98%. Nitrogen dioxide and water vapor thus recovered arerecycled back to the reactor.

The gases leaving the sulfuric acid scrubber consist of chlorine, anyexcessive ogygen and possibly atmospheric nitrogen. In order to recoverchlorine therefrom, the gases are scrubbed with carbon tetrachlorideunder pressure. This scrubbing process may be assisted by cooling downsaid carbon tetrachloride. However it is practical to work at ambienttemperatures without cooling, since complete absorption can be achievedby increased pressure alone, whereby gauge pressures between 40 and 500lb./sq. inch are suflicient. The not dissolved gaseous constituents,consisting of oxygen and possibly nitrogen are vented. Following thisstep the solution of chlorine in carbon tetrachloride is pressurereleased, whereby chlorine is recovered in gaseous form. In this step itis advisable to release or lower the pressure to a gauge pressure lessthan 40 lb./sq. inch, preferably between 10 lb./sq. inch and 30 lb./sq.inch. Complete pressure release to normal pressure might result inremarkable losses of carbon tetrachloride, carried along by gaseouschlorine liberated. When the pressure is released to between 10 and 30lb./sq. inch, losses of entrained carbon tetrachloride are vanishinglylow in consequence of the decreased specific volume of chlorineliberated.

In performing my process, an aqueous solution of hydrochloric acid ofany concentration between 10% and 35% contents of hydrogen chloride maybe used. This may be performed in an apparatus, shown in the annexeddrawing. Referring to this, said aqueous liquid hydrochloric acid is fedin by line 1 into reactor 10. Instead of one reactor 10 two equalreactors may be installed in series, one above the other. Part of theliquid feed is passed via necked trap 4 through the heat exchanger 5 andentering via necked trap 6. Another part of the hydrochloric acid feedis passed via valve and line 3 and then through junction point 8 andnecked trap 9 into reactor 10. A third part of the hydrochloric acidfeed is passed via valve and line 2 through heat exchanger 12 and thencevia line 7 to junction point 8, necked trap 9 and finally into reactor10. Heat exchanger 5 is heated up by the vapors, carried along with thegaseous constituents, which are issuing from top of reactorll). Heatexchanger 12 is heated up by the hot exhausted liquid hydrochloric acid,which issues from the bottom of reactor 10 through necked trap 11, heatexchanger 12 and outlet 13. Reactor 10 is heated up with steam by meansof a tantalum coil, mounted near the exit point at the bottom of reactor10. Tantalum tubings may likewise be installed in heat exchangers 5 and12, as feed pipes at 9 and 6 as well as at position 115. Graphite, cokeor any kind of carbon, molded by means of phenoplasts are likewisefeasible material for said heat exchangers and pipings. Heat maylikewise be supplied to reactor 10 by means of electrically heated rodsof graphite or coke.

A small part of the exhausted liquid, flowing out of 13, may be recycledto points 9 or 6, in order to provide a zone of dilution of hydrochloricacid at the top of the reactor 10 in order to complete hydrolysis of anyentrained nitrosyl chloride. However it was found, that, whenmaintaining reactor 10 at the prescribed temperatures, no more nitrosylchloride could be detected in the gases, leaving reactor 10. The gaseousmixture issuing from the top of reactor 10 passes through heat exchanger5', that acts likewise as a reflux condenser. From the top of 5 thegaseous mixture passes through pipe 17 to scrubber 18. Here nitrogendioxide and possibly some nitric acid are absorbed along with moistureby sulfuric acid. Said sulfuric acid, loaded with nitrogen dioxide,possibly nitric acid and moisture are passed through necked trap 19 toregenerator 20. Here theloaded sulfuric acid is heated by a steam coil,whereby nitrogen dioxide, possibly nitric acid and water are removed andfed back through lines 16 and into reactor 10, by means of a perforatedcoil, located inside of 10. At any point before the entrance of 15 intoreactor 10, air or oxygen are admixed via inlet at needle valve 14.Regenerated sulfuric acid from vessel 20 is passed through necked trap21, condenser 22 and necked trap 23 into scrubber 18, where regeneratedsulfuric acid, .fed in by means of a perforated coil inside 18. Scrubber18 is provided with packing material of any vitreous or graphitecontaining packing material, similarly as reactor 10.

The gaseous mixture issuing from top of scrubber 18 consists of chlorineand possibly oxygen and nitrogen. These gases are passed through line 24to pump 25, where they are compressed to any pressure as stated above.Subsequently they are forced into scrubber 26, which is supplied withcarbon tetrachloride, that extracts chlorine. Any nitrogen or oxygen,which are insoluble in carbon tetrachloride, are passed through needlevalve 32 and thence vented. Carbon tetrachloride, loaded with chlorineis passed through needle valve 27, whereby the pressure is released tobetween 10 and 30 lb./sq. inch gauge pressure in vessel 28. Chlorine,thus liberated, is drawn off on top of vessel 28 and passed throughneedle valve and line 33 to chlorine storage. Exhausted carbontetrachloride, however still containing small amounts of chlorine, isdrawn 0E at the bottom of vessel 28 through necked trap 29 to pump 30.Here carbon tetrachloride is re-eompressed to a pressure of between 40and 500 lb./sq. inch and forced through line 31 into scrubber 26 forre-loading.

Starting and operating the new process is performed as follows: Reactor10 is filled with aqueous hydrochloric acid, and heat is supplied, untilthe temperature inside the reactor 10 has reached about C. Then air oroxygen is fed into reactor 10 via 14 and 15, and in addition to this,nitric acid, amounting to 3%5% of the total amount of hydrogen chloridein reactor 10, is likewise fed in via 14 and 15. The reaction startsalmost immediately. Since the reaction is exothermic, heat supply hasnow to be reduced, so as to maintain said 100 C. Now further aqueoushydrochloric acid is fed in by line I, and the further distribution ofthe liquid feed is distributed through the three feed lines 2, 3 and 4in such a manner, that minimum quantities of moisture are carried awaythrough line 17 and outfiowing exhausted aqueous hydrochloric acid showstemperatures of 50 C. and less.

The gaseous mixture issuing from the top of heat exchanger 5 is passedthrough a sulfuric acid scrubber and then to the carbon tetrachloridescrubber. Regeneration of loaded sulfuric acid from 18 as well aschlorine-loaded carbon tetrachloride from scrubber 26 may be performedcontinuously or batchwise, whilst operation of reactor 10 is performedin a continuous manner. When the apparatus has been started, furtheraddition of nitric acid is in general no longer necessary, since fromnow on reactor is supplied with nitrogen dioxide and possibly somenitric acid from regenerator 20. From time to time a little bit nitricacid will be admitted through 14 for the purpose only, to even upinevitable losses of nitric values.

In the event oxygen is supplied through lines 14 and 15 the carbontetrachloride scrubber assembly might be replaced by a conventionalcompressor and cooling-unit, whereby liquefied chlorine is collectedimmediately. In this event however the chlorine produced containsremarkable quantities of oxygen, which are undesirable. For it preventsvery often starting and operating chlorination processes by interruptingthe valuable reaction chains. In addition to this said compressor andcooling: principle is always burdened with the occurrence of appreciablequantities of so called Sniff Gas, that means a substantiallynon-condensable gaseous mixture of oxygen and chlorine of about 1:1. Ifhowever the apparatus is operated with the by far cheaper air, then thecooperation of carbon tetrachloride assembly 26 is advisable. Inaddition to this chlorine is obtained in a substantially oxygen-freeform, which involves considerable advantage in all chlorinationprocesses.

An apparatus, quite similar to the above described, with a reactor of6.1 feet hight and 6 inches inner diameter was used for the followingrun: An aqueous hydrochloric acid of 20% hydrogen chloride was passedthrough said reactor together with 800 cu. yd. (standard conditions) ofair, mixed with 50 grams of nitrogen dioxide in countercurrent flow. Thetemperature inside the reactor was maintained at 90-92" C. The stay-timeinside the reactor amounted to about 40 minutes. When the apparatus hadreached steady equilibrium, the ex hausted acid, drawn ofi from point 13(see annexed drawmg), contained 2.2% hydrogen chloride, whilst 17.8%hydrogen chloride were completely converted to chlorine. In this run aspace-time efiiciency of 427 lbs. chlorine/ hour/cu. yd. was reached.

As mentioned above the new process is of special value in combinationwith chlorination processes in general. In these chlorination processestremendous quantities of hydrogen chloride are occurring as a mostundesired byproduct, since there is little demand for hydrochloric acid,and that only for chemically pure one. As a rule however hydrochloricacid occurring in chlorination processes is always contaminated and veryhard to purify. Consequently most chlorination processes cannot becarried out commercially, unless hydrochloric acid occuring is recoveredto elemental chlorine. To this end hydrogen chloride occurring inchlorination plants must be absorbed in water and in this form shippedor piped to the chlorine recuperation plant. But since the knownprocesses concern only processing of substantially waterfree hydrogenchloride, said shipped aqueous hydrochloric acid must be processed againto waterfree hydrogen chloride, previous to further processing tochlorine.

Due to the fact that the new process is basic in immediate processing ofliquid aqueous hydrochloric acid of any concentration, this processenables to run the. recuperation of chlorine in an utmost cheap andprofitable manner. To this end hydrogen chloride, occurring in theseveral chlorination plants of a factory, is absorbed in water andshipped or piped to the chlorine recuperation unit. Here said aqueoushydrochloric acid is converted to chlorine according to the new process.In this event however it is advisable to increase the throughput throughthe reactor, as to obtain an outflowing aqueous hydrochloric acid, stillcontaining remarkable amounts of hydrogen chloride, of HCl and less. Ingeneral concentrations of 2 to 5% of hydrogen chloride are advisable.Under these circumstances the spacetime efiiciency of the reactor(position 10 of annexed drawing) can be improved to twice its normalvalue and more. The exhausted, say 4% hydrogen chloride containingoutflow may be passed through a graduation equipment, following outflowposition No. 13, in order to cool down completely to ambienttemperatures and to get rid of that part of water, which is produced inconsequence of the oxidation of hydrochloric acid. Then the dilutedaqueous hydrochloric acid is sent to said chlorination plants forre-loading with hydrogen chloride, followed by re-cycling again the moreconcentrated hydrochloric acid to the chlorine recuperation plant. Bythis way quantitative yields of chlorine recovered are obtained withvery high throughputs, simultaneously preventing any sewage problem.

What I claim is:

1. A process for converting hydrochloric acid into chlorine comprised ofthe following steps: passing an aqueous hydrochloric acid solution ofconcentration not to exceed 35% in countercurrent flow with a gaseousmixture of air and N0 at temperatures between 80 C. and the boilingpoint of said hydrochloric acid solution through a reactor provided withpacking material, scrubbing the efiiuent gases from said reactor withsulfuric acid having a concentration of %-98% to absorb N0 and moisturefrom the said effluent gases, further scrubbing the unabsorbed gasesissuing from the sulfuric acid scrubber with carbon tetrachloride underincreased pressures, and recovering pure chlorine in the gaseous stateby partial pressure release of the carbon tetrachloride solution.

2. A process for converting hydrochloric acid into chlorine comprisedofthe following steps: passing an aqueous hydrochloric acid solution ofconcentration not to exceed 35% in countercurrent flow with a gaseousmixture of oxygen and N0 at temperatures between C. and the boilingpoint of said hydrochloric acid solution through a reactor provided withpacking material, scrubbing the efiiuent gases from said reactor withsulfuric acid having a concentration of 60%98% to absorb N0 and moisturefrom the said effluent gases, further scrubbing the unabsorbed gasesissuing from the sulfuric acid scrubber with carbon tetrachloride underincreased pressures, and recovering pure chlorine in the gaseous stateby partial pressure release of the carbon tetrachloride solution.

3. A process for converting hydrochloric acid into chlorine comprised ofthe following steps: passing an aqueous hydrochloric acid solution ofconcentration not to exceed 35% in countercurrent flow with a gaseousmixture of oxygen and N0 at temperatures between 80 C. and the boilingpoint of said hydrochloric acid solution through a reactor provided withpacking material with increased throughput through the reactor to such arate, that the concentration of the effiuent aqueous liquid hydrochloricacid amounts to 3%-l0%, whereby the latter is recycled for reloadingwith hydrogen chloride, scrubbing the effluent gases from said reactorwith sulfuric acid having a concentration of 60%-98% to absorb N0 andmoisture from the said effluent gases, further scrubbing the unabsorbedgases issuing from the sulfuric acid scrubber with carbon tetrachlorideunder increased pressures, and recovering pure chlorine in the gaseousstate by partial pressure release of the carbon tetrachloride solution.

References Cited in the file of this patent UNITED STATES PATENTS1,310,943 Datta July 22, 1919 2,092,383 Tramm et a1. Sept. 7, 19372,393,229 Bouchard Jan. 22, 1946 2,656,011 Frey Oct. 20, 1953 2,665,195Congdon Jan. 5, 1954 OTHER REFERENCES I. W. Mellors Inorganic andTheoretical Chemistry, vol. 8, 1928 Ed., page 618. Longmans, Green &Co., N. Y.

Roscoe & Schorlemmer, vol. 1, Treatise on Chem., 1905, pages 194,202-204, MacMillan & Co., Ltd., N. Y.

1. A PROCESS FOR CONVERTING HYDROCHLORIC ACID INTO CHLORINE COMPRISES OFTHE FOLLOWING STEPS: PASSING AN AQUEOUS HYDROCHLORIC ACID SOLUTION OFCONCENTRATION NOT TO EXCEED 35% IN COUNTERCURRENT FLOW WITH A GASEOUSMIXTURE OF AIR AND NO2 AT TEMPERATURES BETWEEN 80* C. AND THE BOILINGPOINT OF SAID HYDROCHLORIC ACID SOLUTION THROUGH A REACTOR PROVIDED WITHPACKING MATERIAL, SCRUBBING THE EFFLUENT GASES FROM SAID REACTOR WITHSULFURIC ACID HAVING A CONCENTRATION OF 60%-98% TO ABSORB NO2 ANDMOISTURE FROM THE SAID EFFLUENT GASES, FURTHER SCRUBBING THE UNABSORBEDGASES ISSUING FROM THE SULFURIC ACID SCRUBBER WITH CARBON TETRACHLORIDEUNDER INCREASED PRESSURES, AND RECOVERING PURE CHLORINE IN THE GASEOUSSTATE BY PARTIAL PRESSURE RELEASE OF THE CARBON TETRACHLORIDE SOLUTION.